Dog allergen components

rCan f 1 e101

rCan f 2 e102

rCan f 5 e226

Native allergens

nCan f 3 - Dog serum albumin e221

The dog is a relative of the wolf, the jackal, and the fox; all belong to the family Canidae. Two characteristics distinguish the dog from other canids: its worldwide distribution in close association with humans, and its huge variety as a result of adaptation and breeding for specific purposes. Over the centuries dogs have acquired the body types and dispositions to pursue and retrieve game, and to be draught animals, guides (e.g. for the blind), guards, companions, and so on.

Dogs are found in almost every human environment. Some dogs are feral, but not in such large numbers as cats are.

As with cat, major dog allergens can be found in hair, dander, pelt, saliva and serum, and are considered epithelial allergens; unlike with cat, however, dog urine and faeces do not have any significant allergenic activity. (1, 2, 3, 4, 5) The concentration of allergens varies between and within breeds. (2, 6) Although allergen differences occur according to the source of the allergen (e.g. epithelium or saliva), no breed-specific allergens occur. (7, 8) This is contrary to reports of much earlier studies. (9, 10)

Dog allergens are ubiquitous in the environment. For example, they may be found on automobile seats in concentrations well above the thresholds for both sensitisation and symptoms, regardless of the presence of a pet in the home. (11) Dog allergens are also prevalent on walls, smooth floors, and finished furniture in homes with and without pets, (12) as well as on furnishings and textiles in classrooms. (13, 14) The concentration of dog (Can f 1) allergen may be even higher in dust collected in schools than in that collected in homes. (15) High dog allergen levels can be found in households without a pet if the former occupants had a pet, or if dogs often visit the building. (16)

Upholstered chairs in hospitals constitute a significant reservoir of cat and dog allergens, and inhalation of airborne allergens by patients attending their hospital appointments may exacerbate asthma in those highly allergic to cats or dogs. (17)

The association between pet exposure and asthma or allergic sensitisation can be very confusing, and many conflicting findings have been published. (18) Recent studies can be used to support nearly any viewpoint on the issue: Dog exposure decreases (19, 20) or has no effect (21) on the risk of sensitisation; asthma is negatively (21) or positively (22) associated with dog exposure. What makes certainty impossible is that dog (and cat) allergens are ubiquitous in human society and may affect sensitisation in predisposed individuals, regardless of pet ownership. (18, 23, 24)

Early studies reported that over 28 dog antigens were detected, 11 of which were found in dog serum. IgE antibody in the sera of dog-sensitive patients was reported to bind to 21 of these antigens to varying degrees. (3,) (4)

The following allergens have been characterised:

Can f 1, a lipocalin. (2, 32 ,33, 34, 35, 36, 37)

Can f 2, a lipocalin. (2, 32, 33, 35, 37)

Can f 3, dog serum albumin. (28, 35, 38)

Can f 4, an 18 kDa protein, an odorant binding protein. (33, 35, 39)

Can f 5, also known as prostatic kallikrein, an arginine esterase. (35, 40, 41, 42, 43)

Two serum proteins, alpha-1-antitrypsin and IgG, have been identified as minor allergens. (8)

Can f 1 was originally named Ag13 and was found to be identical to Ag8. (2) Can f 1 is a 22 to 25 kDa protein found in hair, dander and saliva, but not in serum, and is a lipocalin family member. (32)

The amount of dog allergens produced appears to have wide variability among dog breeds. Hair length or hormonal status does not influence the production of Can f 1 (except that males produce more than females), whereas seborrhoea strongly influences the presence of Can f 1 on hair. (2) Older animals produce more dander than younger ones, because their skin is drier. Also, epidermal turnover is more rapid in dog breeds that are prone to the various forms of dry and oily seborrhoea. Instead of the normal 21-day cycle, the epidermal turnover time of seborrhoeic dogs is only 3 to 4 days.

Can f 2, a 19 kDa protein found in dander and saliva, and previously known as Can d 2, is a lipocalin family member and has homology with mouse urinary protein (MUP). (32, 44) In the majority of studies, it is shown to be a minor allergen.

Can f 3, dog serum albumin, a 69 kDa protein, is found in dander, epithelia, saliva, and serum. (37) It has also been found in salivary glands (parotid and submandibular) and liver. (45) Dog albumin represents an important allergen for up to 35% of patients who are allergic to dogs. (28)

Can f 4 is an allergen found in dog dander.

Can f 5, prostatic kallikrein, is found in dog urine, and a closely related or identical protein in dog dander. (39)

Shared IgE epitopes of the major cat and dog allergens may provide an explanation for the clinical observation that allergies to cats and dogs are frequently associated. (46) However, several studies report that actual common allergens are responsible for the cross-reactivity, and that these allergens appear to be serum albumin and lipocalin. Furthermore, in a study of 36 cat-allergic patients, in 25% of Fel d 1-reactive patients more than 50% inhibition of IgE reactivity to dog allergens was achieved with recombinant Fel d 1. A Fel d 1 cross-reactive 20 kDa allergen was detected in dander extracts of several different dog breeds, which may be responsible for double positivity to cat and dog in serology. However, the clinical relevance of this cross-sensitisation was not clinically evaluated. (47)

Importantly, dog-allergic individuals are sensitised to a heterogeneous range of dog allergens. For example, in a study of such individuals, 52% were shown to be sensitised to Can f 1, about 33% to Can f 2, 60% to an 18 kDa protein, 44% to a 40 kDa protein, and 48% to a 70 kDa protein (probably serum albumin, now known as Can f 3). (33)

A second study (evaluating Can f 5) illustrates the heterogeneity of allergen sensitisation, in which recombinant Can f 5 was shown to bind IgE antibodies from 26 (70%) of 37 sera of subjects with dog allergy, 14 of which reacted to none of Can f 1, Can f 2, or Can f 3. (39) Of 37 sera of subjects with dog allergy, 26 (70%) showed IgE reactivity to recombinant Can f 5, and 18 (49%) to rCan f 1. Both rCan f 2 and nCan f 3 appeared as minor allergens among the subjects studied, binding IgE antibody from only 8 (22%) and 6 (16%) respectively. Fourteen (38%) of the 37 sera reacted to rCan f 5 alone. (39) Only 2 of the 37 sera tested showed no IgE reactivity to any of the 4 dog allergens tested.

Recombinant allergens (which are genetically engineered isoforms resembling allergen molecules from known allergen extracts) have immunoglobulin E (IgE) antibody binding comparable to that of natural allergens, and generally show excellent reactivity in in vitro and in vivo diagnostic tests. (48 )To date, many different recombinant allergens of various pollens, moulds, mites, and foods (as well as allergens of latex and bee venom) have been cloned, sequenced, and expressed.

Recombinant allergens have a wide variety of uses, from the diagnosis and management of allergic patients, to the development of immunotherapy, to the standardisation of allergy test products as tools in molecular allergology. Recombinant allergens are particularly useful for investigations of allergies manifesting wide cross-reactivity.

rCan f 1

Allergen

rCan f 1. (32, 33, 37, 49)

Allergen source material

Common names

Ag 13.

Biological function

Can f 1 is a lipocalin.

Mw22-25 kDa.

Allergen description

Can f 1 (originally designated Can d 1) is a lipocalin. Can f 1 is a major allergen and the most important dog allergen, and dog dander and saliva have a high Can f 1 content (though serum has none). The protein is produced in the canine Von Ebner’s glands, which are small salivary glands opening in the lingual epithelium. This protein ranges in size from 21 kDa to 25 kDa. (50, 51) Can f 1 has demonstrated greater heat resistance than mite allergens after 60 minutes at 140°C. (16) The protein is also relatively stable in house dust. (26)

Major respiratory allergens of dogs, mice, rats, horses and cows belong to the lipocalin group of proteins. The amino acid sequence identity between lipocalins is often less than 20%, but they contain between 1 and 3 structurally conserved regions, and their 3-dimensional structures are similar. Lipocalins share certain biological functions, predominantly related to the transport of small hydrophobic molecules such as vitamins and pheromones.

Immune reactivity to lipocalin allergens is not well understood. In Bos d 5, the IgE-binding epitopes are spread along the molecule, whereas in Bos d 2, the C terminus appears to contain the human B cell epitopes. Bos d 5 contains several murine T-cell epitopes. To explain these observations, it has been proposed that the allergenicity of lipocalins may be a consequence of molecular mimicry between lipocalin allergens and endogenous lipocalins at the T-cell level. (51)

Can d 1 and Can d 2 are found in the sera of approximately 74% of dog-allergic individuals. (44) More than 90% of dog-allergic patients have been shown to have specific IgE directed to Can f 1 alone. (2, 32, 52, 53) In another study, sera from 96% of patients with dog allergy demonstrated specific IgE to Can f 1 and Can f 2. Can f 1 was preferentially detected in dander and saliva, but not in skin, salivary gland, serum and liver extracts. Can f 2 was strongly expressed in skin, but not in dander, serum and liver. (1, 25) However, not all studies have found a high prevalence of IgE reactivity in dog-allergic patients; one study reported that, according to ELISA determination, 52% of dog-allergic patients recognised recombinant Can f 1. (33) The authors postulated that this may have been due to their selection of patients, but it has not been determined whether certain populations are less frequently sensitised to Can f 1.

Recombinant Can f 1 and Can f 2 are immunologically concordant with natural Can f 1 in serum- and skin-specific IgE analysis. The concordance is slightly lower with recombinant Can f 2. Fifty-two percent of dog-allergic patients reacted against Can f 1, and about a third of the patients reacted to Can f 2. (33)

As the important allergens in commercial dog extracts can vary extensively, and as natural preparations may be contaminated with Mite allergens, potentially causing false-positive skin-specific IgE test results, recombinant Can f 1 and recombinant Can f 2 have a role to play in assessing allergy to dog. (54)

Can f 1 and 2 are two important and useful tools identified so far, but further components are needed for diagnosing dog allergy. (33)

rCan f 2

AllergenrCan f 2. (32, 33, 37, 48)

Common names

None.

Biological function

Can f 2 is a lipocalin.

Mw

19 kDa.

Allergen description

Can f 2, previously known as Can d 2, is a protein with a molecular weight of 19 kDa (44) or 27 kDa. (4) It is a lipocalin and has homology with mouse urinary protein (MUP). (2) It was found to react with IgE antibodies of 66% of dog-allergic patients, and to bind 23% of the IgE antibodies directed against dog dander extract, both of which findings confirm its role as a minor allergen. (2) Recombinant Can f 2 has a slightly lower concordance compared to natural Can f 2, with about a third of dog-allergic patients found to be sensitised to this recombinant allergen. (33) Can f 1 and Can f 2 share epitopes. (49)

A study evaluated the recombinant Dog allergens Can f 1 and Can f 2 in clinically diagnosed dog-allergic patients and in healthy, non-atopic dog owners. These allergens were compared to commercial dog epithelial extract, and it was found that patients' IgE reactivity to natural Can f 1 and to the recombinant allergen were perfectly concordant, but the concordance was slightly lower with recombinant Can f 2. About one-third of the patients reacted to Can f 2. The study concluded that the recombinant allergens can be used reliably to identify Can f 1 and Can f 2-sensitised individuals, but that on their own the 2 allergens were insufficient as reagents for diagnosing dog allergy. (33)

nCan f 3 - Dog serum albumin e221

Allergen

nCan f 3. (3, 8, 10, 28, 37, 35, 53, 55, 56, 57, 58)

Allergen source material

Native dog serum albumin

Common names

Dog serum albumin, DSA.

Biological function

Can f 3 is a serum albumin.

Mw

69-70 kDa.

Allergen description

Can f 3, also known as Dog Serum Albumin (DSA), is a protein with a molecular weight of 69-70 kDa. It is a serum albumin. It is found in dog serum, saliva, dander, hair and epithelia, and it is also synthesised in the dog salivary gland and dog liver. (37) Dog serum albumin has been reported to be a particularly abundant allergen in dog epithelia extracts. (55) Dog and cat serum albumins are also allergens very commonly present in house dust. (44) A recombinant Can f 3 has been produced. (37)

Sensitisation to dog serum albumin has previously been documented as varying from around 35-48%, although early studies reported even lower frequencies. (5, 28, 33, 59) The importance and frequency of sensitisation to DSA also varies in different populations. (3)

In a study, 51 patients with a clinical history of dog allergy were evaluated for skin-specific IgE for 8 individual, standardised dog breed allergen preparations, and for 1 mixed-breed allergen preparation, dog serum albumin, and histamine hydrochloride. The sensitivity rate shown by skin-prick test was 67% to 88% for the various dog breed allergen preparations, but only 18% for DSA. (10)

The deduced amino acid sequence of DSA was shown to be highly homologous to the sequences of albumins from both other animals and humans, which explained the perceived extensive cross-reactivity among albumins, and was corroborated by the demonstration of the presence of similar epitopes on dog, cat, and human albumin. (37) In immunoblot inhibition studies and histamine release tests, it was demonstrated that patients who react to Dog albumin exhibit IgE reactivity with purified albumins from cat, mouse, chicken, and rat. The deduced amino acid sequence of DSA was found to have significant sequence homology with albumins from human (82.6%), pig (81.8%), beef (77.3%), sheep (78.8%), mouse (75.8%), and rat (76.2%). (28)

Cross-reactivity between DSA and albumins from other animals was demonstrated in other studies. In a study aimed at assessing the importance of albumin as a cross-reactive allergen in patients sensitised to cat, dog and horse, 117 patients sensitised to cat were tested for the presence of skin- and serum-specific IgE. Twenty-two percent of patients were found to have specific IgE to cat albumin, and 41% of these patients were also sensitised to dog and horse. Of this group, 21% had IgE to all 3 albumins and 17% to 2. However, inhibition studies demonstrated variable degrees of inhibition, suggesting that albumins from these 3 animals share some epitopes that account for the cross-reactivity observed in around a third of patients sensitised to cat, dog and horse, but that more than 50% of specific IgE that cross-reacts between these 3 animals is directed to allergens other than albumin. (60)

Similarly, in a study evaluating the degree and significance of IgE-cross-reactivity to various albumins in 200 patients allergic to animals, it was found that approximately 30% of those allergic to animal hair/dander extracts reacted to albumins from various animals. Although a high degree of sequence homology existed between different animal albumins, a remarkable variability of IgE cross-reactivity was observed, indicating that some patients were sensitised preferentially against certain albumins. Most of the patients allergic to albumins reacted to dog, cat, and horse albumin, which also bound a high percentage of albumin-specific IgE. Recombinant dog albumin fragment, representing 265 amino acids of the mature protein, bound IgE from all 15 patients allergic to albumin who were tested. (61)

An association between allergy to epithelia and allergy to mammalian meat has also been reported, and most authors ascribed this to serum albumin as the responsible cross-reacting allergen. For example, a 28-year-old asthmatic male cook sensitised to dog epithelium was reported who developed wheezing and contact urticaria when handling raw beef in an occupational setting. Skin-specific IgE was found to raw and cooked beef and raw lamb, and to cat and dog. Dog serum-specific IgE was positive. The secondary cross-reactivity was attributed to Bovine Serum Albumin (BSA). (62)

Furthermore, patients with persistent milk allergy and specific IgE to BSA were reported to be at greater risk of rhinoconjunctivitis and asthma because of cross-reactivity with serum albumin present in animal dander. In a study evaluating the cross-reactivity between the serum albumin of various mammals in milk, meat, and epithelia, sera from all but 1 patient recognised serum albumin in cow’s milk, in meat from beef, lamb, deer, and pork, and in epithelia from dog, cat, and cow. Some patients were sensitised only to serum albumin in meat and epithelia. Patients allergic to only dander recognised other proteins in epithelia, but not serum albumin. The authors concluded that serum albumin is an important allergen in cow’s milk, meat, and epithelia allergy. The authors proposed that sensitisation first occurs by contact with serum albumin in cow's milk and that patients develop sensitisation to serum albumin present on animal epithelia, even without direct contact with animals. The authors cautioned that patients with both BSA and cow's milk allergy must avoid raw meats and furry pets. (63)

Although studies have demonstrated a high degree of homology among serum albumins from various mammals, epitope diversity results in different clinical expressions of cross-reactivity. This is particularly well described in pork-cat cross-reactivity, where cross-reactivity has been demonstrated between pork meat and cat epithelia as a result of serum albumin, but not with dog. (64, 65)

Dog serum albumin may be used for diagnostic purposes to identify patients who are cross-sensitised to many animal species, and may perhaps be used for specific immunotherapy for sensitised individuals. (37)

rCan f 5

Allergen

Allergen source material

Common names

Prostatic kallikrein, also known as urinary kallikrein or arginine esterase.

Biological function

Can f 5 is an arginine esterase.

Mw

28 kDa

Allergen description

Can f 5, also known as prostatic kallikrein, is a protein with a molecular weight of 28 kDa. The unreduced 28 kDa band appears to be made up of an 18 and a 10 kDa protein. It is an arginine esterase found in dog urine, with a similar or identical protein in dander. (39) A recombinant Can f 5 displaying immunologic and biochemical properties similar to those of the natural protein has been produced. (39)

Prostatic kallikrein has been described as an androgen-regulated protein expressed in the prostate (Chapdelaine 1988 ref. 26113 7), (66) suggesting an occurrence restricted to male animals. Castrated male dogs have been found to have drastically reduced production of prostatic kallikrein. (66) Can f 5 was characterised utilising urine collected from an adult, intact male dog. (39)

This suggests that subjects with dog allergy (depending on their specific sensitisation profile) might be differently sensitive to male and female dogs – in particular for patients sensitised exclusively to Can f 5, as were one-third of the subjects in this particular study. (39) And as male dog castration practice may vary between countries and regions, the importance of allergy to Can f 5 may differ accordingly. Importantly, commercially-available dog dander comprises material from a pool of male and female dogs of different breeds. (39) A simple experiment could not detect Can f 5 from the dander of a female dog, although Can f 1 was readily detected. (39) The authors speculated as to how prostatic kallikrein becomes deposited on dog hair or dander, and whether prostatic kallikrein is consistently absent from the hair and dander of female dogs.

Can f 5 is shown to be a major allergen. Recombinant Can f 5 was shown to bind IgE antibodies from 26 (70%) of 37 sera of subjects with Dog allergy, 14 of which reacted to none of Can f 1, Can f 2, or Can f 3. (39)

Of 37 sera of subjects with Dog allergy, 26 (70%) showed IgE reactivity to recombinant Can f 5, and 18 (49%) to rCan f 1. Both rCan f 2 and nCan f 3 appeared as minor allergens among the subjects studied, binding IgE antibody from only 8 (22%) and 6 (16%) respectively. Fourteen (38%) of the 37 sera reacted to rCan f 5 alone. (39) Only 2 of the 37 sera tested showed no IgE reactivity to any of the 4 dog allergens tested.

On average, in all rCan f 5-reactive sera, the level of IgE-antibody binding to rCan f 5 amounted to 54% of that to dog dander. The corresponding relative levels of IgE antibody binding to rCan f 1, rCan f 2, and nCan f 3 were 45%, 25%, and 47% respectively among sera specifically reactive to those allergens. (39)

Dog prostatic kallikrein (Can f 5) shares structural similarity (55% to 60% sequence identity) with human prostate-specific antigen (PSA). (39) PSA which is present in seminal plasma has been shown to be a key culprit in IgE-mediated vaginal reactions to semen. (67, 68) The binding of PSA to IgE antibody from a patient with allergic reactions to human semen has been demonstrated. (67) Similar results were reported: four PSA-reactive sera were identified and used in IgE inhibition experiments; IgE antibody binding to PSA was extensively inhibited by the dog allergen in 2 of the 4 sera (88% and 98% respectively), whereas partial inhibition (60%) was obtained in 1 serum and no inhibition occurred in 1 serum. Control experiments demonstrated that the IgE inhibition activity of recombinant Can f 5 was immunologic and specific. (39)

It is therefore possible that infertility in humans may be as a result of IgE-mediated reactions to Can f 5. (39)

Although dog prostatic kallikrein shows an intermediate level of sequence conservation (53% to 59% sequence identity) with other members of the same protein family from different species (cow, horse, rat, rabbit, mouse, and guinea pig), a certain degree of cross-reactivity may exist; but the extent of the clinical relevance has not yet been evaluated. (39)

Can f 5 may be used for component-resolved diagnostic purposes for dog allergy, and possibly to identify patients who are cross-sensitised to PSA and experiencing allergic vaginal reactions to human semen and complications caused by that condition, and perhaps may be used for specific immunotherapy for sensitised individuals. (37)

De Groot H, Goei KGH, van Swieten P, Aalberse RC. Affinity purification of a major and a minor allergen from dog extract: serologic activity of affinity-purified Can f I and of Can f I-depleted extract. J Allergy Clin Immunol 1991;87:1056-65.

Almqvist C, Larsson PH, Egmar AC, Hedren M, Malmberg P, Wickman M. School as a risk environment for children allergic to cats and a site for transfer of cat allergen to homes. J Allergy Clin Immunol 1999;103:1012-7.